1. Field of the Invention
The present invention relates to an apparatus for wrapping an article with a stretch film and a method therefor.
2. Description of the Related Art
As a conventional method of wrapping an article, a method for wrapping an article with a shrinkable film has been used, which comprises heating the article through a heating tunnel or the like and then wrapping the article by a shrinking force cause by heating. This method required a heating means and thus has raised the problem that a large heating tunnel is required for a large article, resulting in incurring high film and system costs.
In place of this method, a method for wrapping called pallet or stretch wrapping has been known and used in recent years. This method involves winding a film, while stretching it, around articles, or grass or a roll of paper stacked on a pallet, thus wrapping the article. In recent years, this wrapping method has been developed and contributed to the rationalization of transportation, storage, wrapping of articles. A wrapping method now in prevalent use, comprises longitudinally stretching a film with a winding tension exerted on an article and then wrapping the article with the shrinking force of the stretched film. There is also provided another method for wrapping an article comprising longitudinally stretching a film and then wrapping the article with an appropriate winding tension (Japanese Patent Publication Nos. Sho 59-52087 and 61-27254). A stretch wrapping (pallet wrapping) system does not require a heating means such as a heating tunnel. Since this system requires a less expensive apparatus cost, is suitable for wrapping a large article and has thus come to be widely used in recent years.
According to these conventional wrapping methods, in order to wind a film around the entire side surfaces of an article, the film must be wound to form multiple plies while they partially overlap each other because the film has a width which is smaller than the height of the article. When a normal 500-mm wide film is used, the film must be wound in 10-15 plies around an article having a height of 1 to 1.5 meters.
In order to reduce the number of turns or plies, a film having a larger width has been considered. However, a wide roll film is inconvenient for handling during its transportation and for mounting it on a wrapping apparatus. It is difficult to uniformly wind, in a roll-like manner, a wide thin (about 25 .mu.m) film with one or both surfaces being coated with an adhesive. It is also difficult to manufacture a wide film.
In order to generally reduce the amount of the film used, a film having a thickness as small as possible is desirably manufactured to reduce the wrapping cost. A film tends to be very stretchable and is coated with an adhesive. It is, therefore, difficult to manufacture a film having a thickness of 10 .mu.m or less whether it is large or small in width.
A lateral stretch system generally used in the film industry has been considered to stretch a film having a small width laterally to obtain a wide film. Any lateral stretch system, however, is not always suitable for pallet wrapping. For example, as lateral stretch apparatuses generally used in the film industry, a tenter apparatus and a tubular biaxial stretch apparatus are available. These apparatuses are expensive, require a large installation floor area, and are not practical for pallet wrapping needing simplicity and convenience. In a tubular biaxial stretch system, production lines must be stopped every wrapping cycle, and it is, therefore, difficult to maintain the internal pressure of a tube constant. It is also difficult to cut a soft, adhesive tubular film and develop it into a flat sheet. The tubular biaxial stretch system requires a certain running distance to develop the tube into a sheet and results in a bulky apparatus.
A variety of other easy-to-handle lateral stretch apparatuses in addition to the above apparatuses are proposed in the film industry. Of these apparatuses so proposed, hardly any ones are in practical use because it is difficult for them to carry out uniform stretching difficult. Slight stretch irregularity of a biaxial stretch film may cause a winding strain. A film having a winding strain results in a defective product.
A known easy-to-handle lateral stretch apparatus is a lateral stretch apparatus using a pair of rollers each having a roll surface constituted by a combination of protrusions and recessed grooves (Japanese Patent Publication No. Sho 46-11599 and others). According to this apparatus, a lateral stretch magnification is small, and stretching is performed at a maximum magnification of about 1.5 times. In particular, if a roll film to be used in pallet wrapping is stretched at about room temperature, a large shrinking stress will be caused. For this reason, the roll film will immediately shrink after being stretched, and, therefore, the effect of lateral stretching is low. Even if the film is stretched laterally, the film is not stretched in the widthwise direction; there is therefore needed an independent step of stretching the film in the widthwise direction.
Another conventional easy-to-handle stretch apparatus is a stretch apparatus utilizing pulleys (BP 849,436). The present inventors have proposed a variety of pulley type stretch apparatuses (e.g., Japanese Patent Publication Nos. Sho 61-55456 and 57-30368). Still another conventional easy-to-handle stretch apparatus is exemplified by a stretch apparatus having a large number of grippers (Japanese Patent Publication No. Sho 63-44049) and needle-like pins planted in the periphery of a rotary disc. Such a conventional rotary disc type lateral stretch apparatus cannot be applied to pallet wrapping because this apparatus allows a laterally stretched film to run horizontally in its longitudinal direction.
The conventional wrapping methods described above are excellent in air-tightness because articles are wrapped with films. For this reason, these wrapping methods are often not suitable for articles such as vegetables and fruits in the fields requiring air or moisture permeability and the fields requiring prevention of dew condensation. As a manufacturing problem, the film width is reduced after longitudinal or lateral stretching, thereby reducing wrapping efficiency. In particular, in lateral stretching, a remarkable widthwise shrinkage occurs after the film is stretched laterally.
There is also available a conventional method using a net-like material as an air-permeable wrapping material and stretching this material along its longitudinal direction. This method, however, results in higher cost than a method using a film. In addition, when an article is large in size, a greater number of turns is undesirably needed for wrapping the article because a net width is small. Therefore, the method using the net-like material is much less practical due to the consumption of a large amount of the material and a long wrapping time.